1. Field of the Invention
Corn fiber is an abundant low-valued by-product of corn wet- and dry-milling industries, and for many years there has been an interest in developing more valuable products from it. The corn fiber gum (hemicellulose B) fraction is the most unique of the carbohydrate components of fiber and is potentially very useful. For corn fiber gum to attract significant commercial interest, however, it should be colorless and lack the flavors and aromas normally associated with corn. This invention pertains to a process for the preparation of corn fiber gum which optimizes a yield of a white, high quality product.
2. Description of Related Art
For many years there has been an interest in developing more valuable products from corn fiber. Hemicellulose is the major component of corn fiber (38-57%), followed by starch (10-30%), protein (15%), cellulose (15%), oil (about 3.7%) and other substances such as lignin and ash (3%), (Osborn and Chen, (1984) Starch/Staerke. vol. 36, pp. 393-395; Carlson, T. (1994) In: Proc. Corn Utilization Conference V. St. Louis, Mo.). The actual yields of corn fiber gum from corn fiber and bran, even on a dry and starch-free basis, are quite variable and largely a function of the conditions used for its isolation.
Early corn fiber gum was prepared (Wolf, et al. (1953) Cereal Chemistry. vol. 30, pp. 451-470) by boiling corn fiber for one hour at pH 10.5-11.5 followed by hot water extraction. Later efforts to lighten the color of corn fiber gum resulted in extractions using sodium or calcium hydroxide, or combinations of the two, at various temperatures. Extracts using calcium hydroxide were the lightest in color and were further lightened by treatment with activated carbon. Yields in such cases were about 27%, and the hemicellulose A and B fractions were not separated. In addition, several processes for producing the arabinoxylan (hemicellulose B) fraction of fiber have been described in the patent literature, using various conditions of alkaline extraction (Wolf, supra; Rutenberg and Herbst, U.S. Pat. No. 2,801,955, 1957; Watson and Williams, U.S. Pat. No. 2,868,778, 1959; Schweiger, U.S. Pat. No. 3,716,526, 1973; Antrim and Harris, U.S. Pat. No. 4,038,481, 1977).
Later, Gould (U.S. Pat. No. 4,806,475, 1989) described a process for producing cellulosic products by treating nonwoody lignocellulosic agricultural byproducts with an aqueous solution of strong alkali and hydrogen peroxide. The lignin portion was thus solubilized, thereby enabling the separation of the non-soluble cellulosic portion from the soluble lignin portion. Chou, et al. (U.S. Pat. No. 4,957,599, 1990) also described a process for delignifying and bleaching lignocellulosic material. The material was first treated with an alkaline solution free of peroxide, then an alkaline peroxide solution. Vail (U.S. Pat. No. 5,057,334, 1991) described the preparation of a cellulose product by treating legume hulls with caustic alkaline oxidizing agent to solubilize the non-cellulose material, then dispersing the resulting pulp in water to form a slurry, lowering the pH of the slurry, then treating the slurry with aqueous hydrogen peroxide. In these efforts, peroxide served to oxidize lignin to low molecular weight organic acids, thereby allowing cellulose to become accessible to the activity of cellulase. The objectives of these methods were to convert renewable lignocellulosic biomass to a form useful for glucose and ethanol production and for use as an energy source in ruminant feeds. To this end, success was achieved in minimizing loss of the hemicellulose fraction during alkaline peroxide extraction of plant sources.
Hemicelluloses are generally defined as polymers that are solubilized from plant cell walls by alkali (Darvill et al. 1980. Pages 91-140, in: The Biochemistry of Plants. P. K. Stumpf and E. E. Conn, eds. Academic Press, New York), and those from corn fiber are typically composed of D-xylose (48-54%), L-arabinose (33-35%), galactose (5-11%), and D-glucuronic acid (3-6%) (Whistler and BeMiller (1956) J. Am. Chem. Soc. vol. 78, pp. 1163-1165; Sugawara, et al. (1994) Starch/Staerke. vol. 46, pp. 335-337; Saulnier, et al. (1995a) Carbohydr. Polym. vol. 26, pp. 279-287; Saulnier, et al. (1995b) Carbohydr. Res. vol. 272, pp. 241-253). Most of the fraction is soluble in water after alkaline extraction. Their isolation is actually a two-stage process, involving alkaline hydrolysis of ester linkages to liberate them from the lignocellulosic matrix, followed by extraction into aqueous media. It is thus expected that corn fiber gum is cross-linked to other cell wall components for several reasons. Both ferulic acid and p-coumaric acid are esterified to cell wall polysaccharides in various grasses (Mueller-Harvey et al. 1986. Carbohydr. Res. vol. 148, pp. 71-85). Partial acid (Yoshida, et al. (1990) Agric. Biol. Chem. vol. 54, pp. 1319-1321) and enzymatic (Ohta et al. (1994) J. Agric. Food Chem. vol. 42, pp. 653-656) hydrolysis of corn fiber gum yields oligosaccharide fragments in which arabinosyl units are esterified at primary hydroxyl groups with ferulic acid. Some were esterified with diferulic acid (Saulnier, et al. supra) and acetyl esters were also identified on the arabinoxylan (Saulnier, et al. (1995a), supra). In addition, there is evidence to suggest that esterified ferulic and p-coumaric acids serve to couple lignin and polysaccharide (Helm and Ralph (1993) Carbohydr. Res. vol. 240, pp. 23-38) and that polyphenolics (including lignin) can form alkali-resistant linkages with the hemicellulose fraction of the matrix polysaccharides (Morrison, I. M. (1974) Biochem J. vol. 139, pp. 197-204; Fincher and Stone. (1986) pp. 207-295 In: Adv. Cereal Sci. Technol. VIII. Am. Assoc. Cereal Chem.: St. Paul, Minn.). Ether linkages are present in lignin, and there is evidence that ether linkages are also involved in linking lignin to hemicelluloses (Watanabe et al. (1989) Agric. Biol. Chem. vol. 53, pp. 2233-2252; Hatfield, R. D. (1991) Pages 285-313 In: Forage Cell Wall Structure and Digestibility. Jung et al., eds. ASA-CSSA-SSSA: Madison, Wis.). As a result, most previous preparations of corn fiber gum samples probably contained remnants of lignin, contributing to off-colored products. Protein was also possibly present since stable linkages between hemicellulose and protein in corn bran (Saulnier, et al. (1995a), supra) and rye bran (Ebringerova, et al. (1994) Carbohydr. Res. vol. 264, pp. 97-109) have been identified.
Various optimization studies have been conducted to obtain useful high quality corn fiber gum in high yields. In preliminary experiments, corn fiber gum was isolated by standard extraction methods using saturated Ca(OH).sub.2 as extractant at 70.degree. C. for sixteen hours (Rutenberg and Herbst, supra) or under reflux conditions for one hour (Watson and Williams, supra). The 70.degree. C. extractions produced tannish products. Yields ranged from 28.2 to 35.1% for extractions at 70.degree. C. and under reflux conditions, respectively.
At alkaline pHs, solutions of corn fiber gum isolated by the traditional approaches turned intensely yellow, and the color could not be removed by dialysis. This was likely due to the presence of lignins or proteins which were retained even during extraction at elevated temperatures.
Lignin fragments not removed from corn fiber gum with alkali were believed to contaminate corn fiber gum preparations, resulting in the undesirable color. A method was developed for delignifying agricultural residues (Gould, J. M. (1984) Biotechnol. Bioeng. vol. 26, pp. 46-52) to maximize their digestibility by ruminant animals. This method incorporated hydrogen peroxide in the extraction medium, which is capable of converting the lignin portion into soluble, low molecular weight organic acids. It was shown (Gould, J. M. 1985b. Biotechnol. Bioeng. vol. 27, pp. 225-231) that delignification is most effective at about pH 11.5, the pK.sub.a for the dissociation of hydrogen peroxide, and that the concentration of the species active in delignification, .OH and .O.sub.2, are optimal at pH 11.6.
Surprisingly, it has now been discovered that high quality corn fiber gum may be efficiently produced by hydrogen peroxide treatment of unmilled corn fiber during alkaline extraction and/or by hydrogen peroxide treatment after obtaining the alkaline extract of milled corn fiber.